This invention is related to methods for chemically treating stored quantities of agricultural products, such as grain. The invention is particularly related to treatment methods wherein the circulation of air is utilized to distribute a gaseous chemical.
Agricultural products, such as grain, are frequently stored for a period of time, such as between harvesting and further processing of the products. This storage can last for considerable periods of time. Consequently, in order to maintain the quality of the stored product, certain procedures have been followed in the storage industry to keep the product in good condition and prevent deterioration.
The temperature and humidity of the stored product, for example, must be maintained within certain limits to prevent spoilage. This conditioning has sometimes been accomplished by physically turning the grain within the storage facility. Temperature and humidity control has also been implemented by using large fans to aerate the product through a system of vents in the storage container, typically including an aeration manifold underneath the stored product and vents in the roof or upper structure (commonly known as the "overhead") of the storage facility. In this manner, external air may be forced upwardly or downwardly through the volume of stored product.
In addition to the problems caused by excessive temperature or humidity conditions, stored agricultural products are also susceptible to damage from various live pests, such as insects, which eat the product, lay eggs in it, etc. Consequently, as soon as such products are stored, measures must be taken to prevent the substantial degradation which can otherwise occur. Various liquid chemicals have been applied to stored agricultural products to kill such pests and prevent their damaging the stored product. Such liquid fumigants are applied to the top of the stored product, the chemicals being intended to flow downwardly into the product and throughout the volume of the product to reach infestations in all locations. Such liquid applications, however, are an expensive means of control and are difficult to apply so as to ensure a uniform distribution of the chemical throughout the stored product, which is necessary to eradicate an acceptably high percentage of the infestation.
Gaseous fumigants have also been used for this purpose. A gaseous fumigant may be applied through a forced air ventilation system to circulate the fumigant through the stored product and then out through the vent system. This method is known as the "one pass" system of fumigation. In an attempt to further improve the efficiency of the fumigation operation and the uniformity of gaseous fumigant distribution, a recirculating forced air fumigation method was developed in the art. In this method, any existing vents in the storage container are sealed off from the external atmosphere. An air duct is attached to the container above the level of the product (the storage "overhead") and connected to the intake for a fan or blower which supplies air to an aeration system underneath the stored product. The gaseous fumigant is introduced into the duct or into the container and the fan or blower is utilized to force air and the gaseous fumigant through the stored product. The gaseous air and fumigant mixture is then routed to the intake of the blower by the air duct, and is recirculated through the product, the recirculation being continued for a period of time sufficient to achieve an evenly distributed concentration of fumigant throughout the volume of the stored product.
In practicing such circulating treatment methods, the total quantity of fumigating chemicals is typically released over a relatively short period of time of approximately 10 to 40 minutes. Such short release times necessitate the use in this technique of air flow rates which are relatively high in order to achieve a uniform distribution of the fumigant. The distribution of the fumigant is further affected by the chemical properties of the particular fumigants utilized. Such chemicals are subject to sorption by the stored product, i.e., the chemical may be absorbed into a grain or it may be adsorbed by the surface of the grain. Furthermore, some chemical fumigants will break down into other compounds after application. These factors tend to cause an unbalanced concentration of fumigant, with the highest concentrations occurring at locations nearest the point of release of the fumigant. When such constraints are taken into consideration, the air flow rates required in circulating fumigation methods typically are between 0.01 cfm/bu (cubic feet per minute per bushel of stored product) and 0.2 cfm/bu, which flow rates correspond to effecting one complete change of air through the stored product in between 2.5 and 50 minutes. Lower air flow rates have not been used, because it has been found that a less than totally effective kill of the pests will be obtained with lower air flows.
Relatively high capacity ducts and blowers must thus be provided in the traditional practice of this method, the ducts typically ranging between 12 and 36 inches in diameter and the blowers used requiring 5 to 100 horsepower motors. Such large ducts are relatively expensive and the size of the blowers necessitates a relatively large amount of energy use during such traditional fumigation operations.
The flow rate, as mentioned above, is normally expressed in terms of cubic feet per minute per bushel of product (cfm/bu). In this terminology, agricultural products such as grain can be dried with a flow rate of approximately 0.05 cfm/bu. (corresponding to a 10 minute total air change), cooling and conditioning is accomplished with a flow rate of about 0.2-0.05 cfm/bu. (2.5 to 10 minute air change), and fumigation utilizing the recirculation or one pass forced air methods has traditionally been implemented with flow rates ranging approximately between 0.01 and 0.4 cfm/bu., commonly at 0.025 cfm/bu. (corresponding to a 20 minute air change).
One particular fumigating chemical, aluminum phosphide, which may be described as a solid fumigant, is available in the form of tablets, pellets, or bagged powder. Gaseous hydrogen phosphide (phosphine) is generated from solid aluminum phosphide in the presence of atmospheric humidity. Hydrogen phosphide has been used in the past as a fumigant in static applications, but those skilled in the art have maintained that it should not be used in forced air systems. In its solid form it has been applied into grain as the grain is moved from one container to another, it has been distributed onto the top or top and bottom of the grain within a container, and it has been probed into the grain at various depths. Each of these fumigation methods has relied on the penetrating ability of phosphine gas and the convectional currents within a storage facility to provide distribution throughout the stored product. On occasion, aeration systems with high air flows have been used in an attempt to assist in the penetration of the phosphine. Whereas other fumigants will release and achieve a peak concentration of gas in approximately 10 to 40 minutes, however, aluminum phosphide requires a much longer time, from 16 to 30 hours, to release the phosphine gas contained therein, and the correlation between this release time and the proper air flow rate has not heretofore been recognized.
Therefore, a need has developed for a treatment method for grain and other agricultural products which will provide an even distribution of chemicals at air flows related to the release of the gas and a uniformly high rate of kill without the need for large and expensive air recirculating equipment or the need for an excessive amount of chemicals.